A conventional glow plug disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-324141 includes a shaft-like heater having an embedded resistance-heating element (resistance-heating wire), which generates heat when electrically energized; a metallic, tubular member in which the heater is fixedly provided through press fitting, shrink fitting, brazing, or the like such that a distal end portion of the heater protrudes from the metallic tubular member; and a tubular, metallic shell in which the metallic, tubular member is fixedly provided such that a distal end portion of the metallic, tubular member protrudes from the metallic shell. In the glow plug, two electrodes (terminals) that are connected to the resistance-heating element (resistance-heating wire) via respective junction wires are embedded in the heater in such a manner as to be exposed at a rear end portion of the heater. To one of the two electrodes is connected a lead wire that is connected to a center rod protruding from a rear end of the metallic shell while being electrically insulated from the metallic shell. In the glow plug, an end of the lead wire is connected to a tubular, metallic piece, and the tubular, metallic piece is shrink-fitted to the heater at a portion where the electrode is exposed, thereby establishing connection between the lead wire and the electrode. Meanwhile, the inner surface of the metallic, tubular member is in press contact with the other electrode, thereby establishing connection between the electrode and the metallic shell.
Such a glow plug is inserted into a plug-mounting hole (hereinafter may be called simply a mounting hole) formed in an engine head (cylinder head). A mounting external-thread portion formed on the outer circumferential surface of the metallic shell is screwed into an internal-thread portion formed in the mounting hole. An annular seat face that is tapered similarly to a distal end of the metallic shell is formed at a portion of the mounting hole located deeper toward an engine combustion chamber than the internal-thread portion. The annular seat face limits screwing movement of the metallic shell to thereby position the metallic shell, and establishes gastightness between the interior and the exterior of the combustion chamber. A distal end face (hereinafter may be called an end face) of the metallic shell that is screwed into the engine head is pressed against the annular seat face, whereby the metallic shell is positioned, and gastightness is established.
In the course of use of such a glow plug, heat generated through heat-generating action of the glow plug itself and heat associated with combustion of the engine raise the temperature of not only a distal end portion (a portion corresponding to the resistance-heating element) of the heater, but also a rear end portion of the heater where the electrodes (terminals) are exposed. As mentioned previously, for example, press contact is employed for respectively connecting the electrodes to the lead wire (tubular, metallic piece) and the metallic, tubular member. Subjection of the electrodes to repeated heat cycles in the course of use of the glow plug raises a problem of an increase in the contact resistance of the electrodes. In order to prevent such an increase in contact resistance, heat generated in the course of use of the glow plug must be efficiently released to the engine head so as to avoid excessive temperature increase at a portion of the heater where the electrodes are present. However, in the above-described glow plug and structure for mounting the glow plug, heat of the heater is conducted to the engine head via the metallic, tubular member fitted to the outer circumferential surface of the heater, the metallic shell, and the annular seat face in contact with the distal end face of the metallic shell and formed in the mounting hole, thereby involving a problem of inefficient heat conduction. Furthermore, in the conventional glow plug, since the metallic, tubular member is fixedly provided in the metallic shell through press fitting or the like, the degree of closeness of the joint surfaces depends on the tolerances of the inside and outside diameters of the metallic, tubular member and the metallic shell, surface roughness, and the like. Therefore, thermal conductivity differs among glow plugs, and high thermal conductivity cannot be obtained.
Japanese Patent Application Laid-Open (kokai) No. 2002-276942 (FIG. 2) discloses another glow plug in which a portion of a metallic, tubular member that protrudes from a distal end of a metallic shell and ranges axially from a distal end of the metallic, tubular member to the distal end of the metallic shell has an outside diameter smaller than that of a portion of the metallic, tubular member fixed to the metallic shell; and a distally facing end face is formed in such a manner as to serve as a boundary face between the portions of different outside diameters. In the glow plug, in addition to a distal end face of the metallic shell, a distal end face of a thick-walled portion of the metallic, tubular member can abut the annular seat face in the mounting hole, thereby solving the above-mentioned problem to a certain extent. In other words, the glow plug employs a mounting structure such that a peripheral portion of the distal end of the thick-walled portion of the metallic, tubular member is in direct contact with a dead-end portion (annular seat face) of the engine head, thereby providing good heat conduction efficiency as compared with the above-described glow plug disclosed in Japanese Patent Application Laid-Open No. 2001-324141, in which heat is conducted to the engine head via the metallic shell only.
However, in the glow plug disclosed in Japanese Patent Application Laid-Open No. 2002-276942, both the distal end face of the thick-walled portion of the metallic, tubular member and the distal end face of the metallic shell are in contact with the annular seat face in the mounting hole formed in the engine head. Therefore, the contact area between the metallic, tubular member and the annular seat face decreases by the contact area between the distal end face of the metallic shell and the annular seat face; consequently, direct heat conduction from the metallic, tubular member to the engine head becomes insufficient. A conceivable measure for rendering such direct heat conduction sufficient is to reduce the radial thickness of the distal end of the metallic shell. However, this will raise the following additional problem.
The glow plug is mounted as follows: the metallic shell is screwed into an internal-thread portion of the mounting hole such that the distal end face of the metallic shell is strongly pressed against the annular seat face in the mounting hole, whereby the metallic shell is positioned in a sealed condition. If the radial thickness of the distal end of the metallic shell is reduced, the strength of a distal end portion of the metallic shell against the press lowers, resulting in compressive deformation of the distal end portion or a like problem. In the case where, because of dimensional accuracy, the distal end of the metallic shell protrudes from the distal end face of the thick-walled portion of the metallic, tubular member, when the glow plug is screwed into an internal-thread portion of the mounting hole, the protruding distal end portion of the metallic shell is deformed by a compressive force (reaction force) from the annular seat face, and such a deformation impairs the reliability of seal.
Such a problem will not be involved if the distal end face of the metallic shell is situated rearward of the distal end face of the thick-walled portion of the metallic, tubular member such that, when the distal end face of the thick-walled portion is in contact with the annular seat face in the mounting hole, the distal end face of the metallic shell is located away from the annular seat face. However, in this case, since screwing the metallic shell into the engine head causes the distal end face of the thick-walled portion of the metallic, tubular member to be pressed against the annular seat face, the metallic, tubular member may slide (move) rearward in relation to the metallic shell. As a result, there arises a problem of a loose joint and a defective seal between the metallic shell and the metallic, tubular member.